When the volume changes of a solid specimen are to be measured accurately by liquid displacement dilatometery, while filling the dilatometer, entrapment of a minute amount of air in the system is inevitable. In the case of a dilatometer of a conventional design, e.g., one with a vertical calibrated stem, the compressibility of this air creates a problem in making accurate measurements due to variation of hydrostatic pressure of the liquid in the dilatometer during its operation, especially if the dilatometer liquid is mercury. For instance, this problem is encountered in using such a dilatometer for measurements of expansive properties of concrete and other construction materials.